Using string tuples as key for HashMap [duplicate]

Question

I need Java equivalent for following Python:

In [1]: d = {}
In [2]: k = ("x","2")
In [3]: d[k] = 1
In [4]: print d[("x","y")]

1

Python has tuples which are hashable. I tried following in Java unsuccessfully:

Map<String[], Integer> d = new HashMap<String[], Integer>();
String[] k = new String[]{"x", "y"};
d.put(k, 1);
System.out.println(d.get(k));
System.out.println(d.get(new String[]{"x", "y"}));

It outputs:

1
null

This means reference to String[] is getting hashed instead of the value.

An inefficient way I can think of is concatenating elements from String[] into a single String.

Is there a better way?

Answer 1

HashMaps use Object.hashCode() to create the hash. This, by default, uses a hash of the object that is unique for each instance - but doesn't look into any contents.

You migth want to create a tuple that overrides hashCode() and, in addition to that, is immutable once created:

public class Tuple<T> {
    private final T[] contents;

    public Tuple (T[] contents) {
        if (contents.length != 2)
            throw new IllegalArgumentException();
        this.contents = contents;
    }

    public T[] getContents () {
        return this.contents.clone();
    }

    @Override
    public int hashCode () {
        return Arrays.deepHashCode(this.contents);
    }

    @Override
    public boolean equals (Object other) {
        return Arrays.deepEquals(this.contents, other.getContents());
    }

    @Override
    public String toString () {
        return Arrays.deepToString(this.contents);
    }
}

[Edit]: Note that, if mutable objects are used instead of strings, the getter must perform a deep copy, not just a simple clone() to ensure immutability.

Answer 2

Arrays in Java don't provide hashCode() and equals(Object) methods, so they aren't appropriate as map keys. What you could use instead is Arrays.asList(string1, string1, etc) which would give you an immutable List, which all the methods needed for a Map's key.

Answer 3

You could use Arrays.toString(myArray) as your key.